The present invention relates generally to a magnetoresistive (MR) sensor based on the spin valve effect for sensing magnetic fields, and more particularly, to such a sensor having a laminated, ferromagnetically coupled free layer with specular electron scattering and an improved longitudinal bias for crosstalk noise suppression.
A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
A common GMR read sensor configuration is the spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the spin valve head, while a magnetization direction of the free layer oscillates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the spin valve read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors.
Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic pinning layer to the pinned layer. The antiferromagnetic pinning layer is positioned upon the ferromagnetic pinned layer such that the pinned layer and the free layer form distal edges of the GMR spin valve.
U.S. Pat. No. 5,206,590 (the ""590 patent) discloses a spin valve sensor referred to as a magnetoresistive sensor based on the spin valve effect. The spin valve disclosed in the ""590 patent includes a free layer with a thickness in the range of 50-150 xc3x85. The ""590 patent discloses two longitudinal bias schemes for stabilizing a domain structure of the free layer. One of the schemes is based on an application of hard ferromagnetic films deposited on the edges of the free layer. Alternatively, the ""590 patent discloses that the films deposited on the edges of the free layer may be antiferromagnetic material.
The relatively large thickness of the free layer of the spin valve sensor disclosed in the ""590 patent results in a reduced giant magnetoresistance due to shunting of a sense current, which causes a reduced output signal. The spin valve sensor according to the prior art also has increased sensitivity to crosstalk noise from adjacent tracks recorded on a medium.
A spin valve sensor is disclosed, comprising a free layer, a pinned layer made of ferromagnetic material, a layer of non-ferromagnetic material positioned between the free layer and the pinned layer, and a pinning layer positioned adjacent to the pinned layer such that the pinning layer is in direct contact with the pinned layer. The free layer comprises a multi-layer stack including a non-magnetic insulating spacer positioned between a first and a second ferromagnetic sublayer. The non-magnetic insulating spacer provides a specular electron scattering effect. The first and the second ferromagnetic sublayers each have passive end regions separated by a central active region. The spin valve sensor further includes bias means positioned between the first and the second ferromagnetic sublayers in the passive end regions. The bias means produces a longitudinal bias in the passive end regions of a level sufficient to maintain the central active region in a single domain state.
The spin valve sensor of the present invention provides a larger GMR ratio and less sensitivity to crosstalk noise than prior art spin valve sensors. The increased GMR ratio is provided by initiating a specular electron scattering effect in the free layer, and thereby localizing electrons within a portion of the free layer where a probability of spin-dependent scattering is a maximum. A reduction in sensitivity to crosstalk noise is provided by positioning means for producing a longitudinal bias between end regions of thin ferromagnetic sublayers of the free layer stack.